The present invention relates to electromagnetically energized actuators, and more particularly to an electromagnetically energized actuator configured for minimization of plunger friction and plunger impact.
A conventional pneumatic valve 10 is depicted at FIGS. 1 through 2 in which an electromagnetically energized actuator used to control flow of fuel vapors from a canister to the engine intake manifold. The conventional pneumatic valve 10 includes a housing 12, an electromagnet assembly 14 which includes a solenoid 14a wound on a spool 14b, a brass tube 16 concentrically disposed relative to the spool, a stop 18 fixedly disposed in the tube and composed of a highly permeable material, a plunger 20 reciprocally disposed within the tube and composed of a highly permeable material, a spring 22 of a spring assembly 24 formed at the facing ends 18f, 20f of the stop and the plunger which biases the plunger away from the stop, and a valve seat 26 located on the plunger at an end thereof distal from the aforementioned facing end. The combination of the plunger, the stop, the spring assembly a first plate 32 and a second plate 34, as shown at FIG. 2, constitutes a conventional electromagnetically energized actuator 38.
When the solenoid is energized, the plunger is magnetically pulled toward the stop, overcoming the biasing by the spring such that the facing end of the plunger moves toward the facing end of the stop, with the consequence that the valve seat is opened. When the solenoid is de-energized, the spring pushes the plunger away from the stop, thereby again closing the valve seat.
Energization/de-energization of the solenoid 14a is current (voltage) source 28 regulated, for example, by a pulse width modulated (PWM) signal generated from a microprocessor 30, wherein the programming thereof is designed, for example, for enhanced evaporative emission control to meet EPA emissions regulations, and provides precision flow at low PWM duty cycles to maintain the correct air-fuel mixture at low engine speeds. Moreover, the pulse width modulation is used to control the flow through the purge system with typical frequency ranges from 8 to 100 Hz.
In the aforementioned example, the microprocessor is used to generate the PWM signal of the solenoid so as to supply full voltage to the circuit long enough to allow the solenoid to energize. Once the solenoid is energized, the state of the valve changes from OFF to ON allowing full airflow through the valve (for a normally closed valve), and when the pulse ends the valve returns to its normal state (OFF). The duty cycle determines the percentage of time that the valve is energized providing a way to adjust the flow rate required.
As mentioned, the plunger moves toward the stop due to the magnetic force created by the energized solenoid, wherein the magnetic flux flows through the magnetic package, through the first and second plates and through the plunger and through stop. This change of the position of the plunger regulates, via the valve, the flow of air through the system. When the magnetic force disappears, the spring located between the stop and plunger pushes the plunger against the valve seat with enough force to maintain a good seal at the valve, shutting off the flow when no purge is required.
As plunger moves back and forth (reciprocates) while operating, two main problems emerge.
The first problem relates to the tube 16. Friction between the plunger and the tube occurs as the plunger slidingly reciprocates therealong. The tube is composed of brass which wears over a lifetime of millions of reciprocation cycles. This wear leads to reduced durability and adversely affects the over-all reliability of the pneumatic valve. A conventional redress to this wear problem (and also to prevent tube scratching and oxidation of the plunger) is to TEFLON(copyright) coat the plunger using, for example, EMRALON 334. However, this is an extra manufacturing step which does not eliminate the problem. Problems related to the brass tube include wide tolerances, plunger sliding noise, and corrosion. Additionally, as can be seen best at FIG. 2, the tube 16 is mounted to opposing first and second plates 32, 34 with respect to the housing 12. At the first plate 32, the tube has a thinned end 36 having a uniformly reduced cross-sectional thickness, which involves yet another added manufacturing step.
The second problem relates to plunger impact. With each reciprocation, the plunger is caused by the co-action of the solenoid and spring to impact at its ends of travel. This impacting results in undesirable noise generation and undesirable wear when the plunger hits the valve seat and the stop. This condition is made worse at low temperatures (ie., below about xe2x88x9210 degrees C.). Related to this problem is the excessive amount of magnetic energy stored in the solenoid. This arises because the secondary air gap GS between the first plate 32 and the plunger 20 is constant (basically being about the thickness of the thinned end 36) even as the plunger reciprocates, the only factor affecting the reluctance is the change in area of the plunger within the first plate 32, which area change is not significant enough to substantially increase the reluctance at the secondary air gap, wherein the relation for reluctance, A, at the secondary air gap, GS, is given by:
=l/xcexcA, 
where xcexc is the permeability of the secondary air gap, l is the uniform separation distance between the plunger and the first plate which coincides with the thickness of the tube sidewall at the thinned end 36, and A is the area of the plunger within the first plate). With the reluctance at the secondary air gap GS remaining about the same, and with the reluctance at the primary air gap GP between the facing ends of the plunger and the stop, the stored magnetic energy causes the plunger to impact forcefully and also tends to retard the ability of the spring to effect fast valve closure, and also tends to retard the ability of the magnetic field to effect fast valve opening.
Accordingly, what remains needed in the art is an electromagnetically energized actuator which has low friction, low wear, low impact, and low noise attributes.
The present invention is an improved electromagnetically energized actuator featuring low friction, low wear, low impact and low noise attributes.
The improved electromagnetically energized actuator according to the present invention includes a polymer tube, a stop fixedly disposed in the tube and composed of a highly permeable material, a plunger reciprocally disposed within the tube and composed of a highly permeable material, a spring assembly including a spring which is formed at the facing ends the stop and the plunger which spring serves to bias the plunger away from the stop, a first plate composed of a highly permeable material which is connected to a first end of the tube, and a second plate composed of a highly permeable material which is connected to a second end of the tube.
In an example of an environment of operation, the electromagnetically energized actuator according to the present invention is disposed in a pneumatic valve including a housing, an electromagnet assembly including a solenoid, and a valve seat. The first and second plates are connected to the housing and form part of the electromagnet assembly, and the valve seat is located at an end of the plunger distal from the aforementioned facing end thereof. The plunger is reciprocal between a first position (responsive to biasing by the spring) and a second position responsive to energization of the solenoid (which overcomes the biasing of the spring). The electromagnetically energized actuator according to the present invention may be used in devices other than a pneumatic valve, which is merely presented herein as an exemplification of use.
The tube is composed of a polymer which (relative to a conventional brass tube) allows tighter tolerances between the plunger and the tube resulting in lower wear, less sliding noise, reduction in corrosion, and elimination of thinned end of the tube. The preferred tube polymer is VESPEL(copyright) (a polyimide material) with TEFLON(copyright) (a fluoropolymer resin material) mixture (most preferred), or VESPEL, both are trademarks of and available through The DuPont Company of Wilmington, Del. 19880. These polymers have very good mechanical and chemical properties, including a favorable wear rate and thermal coefficient of expansion, as detailed in xe2x80x9cUsing Advanced Materials to Improve Automotive Part Life by Richard Van Ryper, The DuPont Company, August, 1996, hereby incorporated herein by reference. For example, an advantage of these polymers, particularly the VESPEL with TELFLON mixture, is self lubrication which tends to protect the plunger in such a manner that the existing plunger coating material could be changed to nickel resulting in tighter tolerances capability, lower friction forces resulting in less plunger wear and higher corrosion resistance, with the attendant advantage of long term failure-free performance. The reduced clearance between the tube and the plunger also improves alignment of the plunger with the valve seat, reducing wear and noise.
The polymer tube is partly received into a recess formed at an opening of the first plate, wherein the non-recessed portion of the opening is proximally spaced from the plunger when it is located at its first position. Because of the recess, a compound secondary air gap is thus formed at the opening, including a first gap component at the non-recessed portion of the opening across a small spacing separating the first plate from the plunger, and a second gap component between the first plate and the plunger at the recess portion of the opening across the thickness of the sidewall of the tube. At the first position, the magnetic energy through the plunger is large when the solenoid is energized, thereby delivering an initially high magnetic force on the plunger to overcome the spring biasing. However, as the plunger moves toward the stop, the area of the plunger proximate the first plate at the compound secondary air gap becomes smaller, whereupon the reluctance of the magnetic circuit increases at the compound secondary air gap even as the reluctance between the plunger and the stop at the primary air gap is rapidly decreasing as the plunger comes into proximity with the stop. The moderation of the magnetic circuit reluctance keeps the magnetic energy from increasing rapidly and thereby dampens the plunger impact at the stop, for example by 35% over the conventional electromagnetically energized actuator shown at FIG. 2. This also has the benefit that duty cycles of the solenoid energization will be more efficient: more magnetic force initially will ensure a rapid, firm start of reciprocation of the plunger when at the first position, whereas as the magnetic circuit energy dissipates with increasing secondary air gap reluctance as the plunger reaches the second position. Accordingly, the spring response time is improved, whereby the plunger is returned more quickly upon de-energization of the solenoid.
Accordingly, it is an object of the present invention to provide an electomagnetically energized actuator featuring low friction, low wear, low impact and low noise attributes.
This and additional objects, features and advantages of the present invention will become clearer from the following specification of a preferred embodiment.